Warning device using power tool residual kinetic energy

ABSTRACT

A warning and braking device for use on a power cutting tool such as a circular saw or a lawnmower comprises a generator, a drive shaft and a pulley connected by a lever to the power tool on/off switch. When the power tool is in the working &#34;on&#34; position, the generator assembly does not contact the power tool drive belt and no electric current is produced by the generator. When the power tool switch is shifted to the off position, however, the pulley on the generator drive shaft is brought into contact with the drive belt, exerting a braking force on the drive belt and producing an electrical signal from the generator which is connected to a warning device such as a light or a siren.

TECHNICAL FIELD

The invention concerns a safety drive for a working machine driven by anelectromotor and displaying a moving tool which is potentiallyhazardous, as well as a input shaft for the tool, an electromotor withan output shaft, a drive mechanism connecting the output shaft with theinput shaft, and a switch to activate the electromotor.

The invention was designed particularly with a view to its applicationin circular saws, but can also be employed for other tools, for example,electric lawnmowers which operate with a rotating blade.

UNDERLYING STATE-OF-THE-ART

Machines of this type, some of which are equipped with very quicklyrotating tool parts, present the danger that after the electrical drivemotor is shut off the tool will continue to run for some time, which mayresult in bodily injury when the tool is handled without due caution.The most recent safety regulations therefore demand that, for example,the circular blade of a table-top circular saw must come to rest within10 seconds after the machine is shut off.

To be sure, there is no lack of technical solutions intended to fulfillthis demand; in reasonably priced work machines, particularly forhousehold work, the incorporation of these costly technical measures canaffect the price at which the machine is sold. In the production ofthese machines an attempt is made to largely employ moderately priced,serial components, which applies particularly to electrical drivemotors; these components are selected from marketable standard seriesand are produced in accordance with all the new technical requirements,thus generally causing a sizeable price increase.

DISCLOSURE OF INVENTION

The objective of the present invention is, therefore, to create a safetydrive for a work machine driven by an electromotor, which safety drivefulfills the demand that the tool part have a short overrun period oncethe motor has been shut off, and which can be constructed from thesimplest and most conventional components.

This objective is fulfilled in a safety drive of the initially describedtype by an electric generator which is provided with a drive element andis positioned in the area of the drive mechanism in such a way that itcan be engaged with the drive mechanism, and by an activating deviceconnected with the switch of the electromotor, which device engages thegenerator with the drive mechanism when the switch is disengaged. Inchoosing the small generator, a series-produced, reasonably priced drivegenerator is preferred, which at the simplest would need to be equippedonly with a clamp and a corresponding belt pulley for suitableinteraction with a drive belt. In the case of circular saws, it is ingeneral desirable for the the saw blade to extend beyond the table platein order to assure the highest possible cutting level with the givingsaw blade. This naturally demands that the axis of rotation of theblade, viewed fro the lower side of the table plate, can be brought asclose as possible to the working surface of the table plate. This ishardly possible with a coaxial positioning of the electromotor, since inany case the motor must have a greater diameter than a belt pulley ofminimum diameter on the tool drive shaft. The use of a belt drive istherefore of added advantage under these circumstances.

The warning light fed by the generator can be advantageously placed onthe blade protecting bonnet of a circular saw, it being possible to useboth the bonnet fastener and the splitting wedge as parts of the currentline to the warning light. On the other hand, it is possible, as is thecase with other working machines, to place the warning light directly inthe operating handle for the motor switch.

In many working machines, including circular saws, it customary todesign the activating element for the motor switch in the form of astick-like switch lever whose upper end projects out of the machineframe. Under the invention it is possible to provide this switch leverwith a corresponding activating rod, by means of which the moveablyfastened generator can be meshed with the belt drive when the switchlever is disengaged.

Since it is generally useful to provide the generator with a certainelastic reset capacity, the danger might exist of the switch lever beingpushed out of its off position, thus disengaging the generator from thedrive belt. In shutting off the motor and engaging the generator, it istherefore necessary in such designs to provide the switch lever with alocked position from which it can be released only through activeoperation of a locking element. A weighted catch, for example, can serveas locking element, as will be described in greater detail below on thebasis of specific embodiments. A locked position of the switch leveralso protects the tool from being turned on unintentionally.

In certain cases of application it may also be useful to replace themechanical activating tool for engaging the small generator with anelectromagnetic actuator connecting the electric small generator withthe drive mechanism by means of the generator's drive element. When afriction clutch is used as drive element the electromagnetic actuatorcan, for example, consist of an additional electromagnetic clutch, bymeans of which the friction clutch can be engaged.

An effective design of the electromagnetic actuator can assure that inexcited condition it does not engage the small generator with the drivemechanism, but rather that the small generator is disengaged from thedrive mechanism when the electromagnetic actuator receives an electricstimulus. This can be achieved, for example, by means of a mechanicalreset spring in the actuator, whose force keeps the small generator inits position against the drive mechanism of the tool when the actuatoris not provided with current. The advantage of this arrangement consistsin the fact that even during current failure or other interruption ofthe electrical supply of the tool the safety device retains itseffectiveness.

The electrical activation of the electromagnetic acutator is directlyconnected with the switch to activate the tool's electromotor.

As an alternative to the safety drive in which the small generator isengaged with the drive mechanism by the activating device after theswitch is released, the safety drive can be equipped with a mechanicalbraking device which is engaged with the drive mechanism by means of theactivating device when the switch is released. This braking device canthen operate, for example, a mechanical siren which gives off a warningsignal. Likewise, when the motor requires braking the kinetic energy canbe employed by the braking device to activate an optical-mechanicalwarning and signal device Here it is particularly useful to have a redsignal panel which becomes easily visible to the operating person whenthe switch is released, which panel is kept visible by the brakingdevice and a rod system until the machine comes to a complete stop.

As compared with an electrical warning and signal device a purelymechanical warning and signal device of this type has the advantage thatit is not susceptible to disturbance under certain unfavorableconditions, such as moisture. It is also cheaper than an electricalwarning and signal device.

If the highest standard of safety is demanded, a combined mechanical andelectric warning and signal device can be employed, providing both aaudible warning tone and a warning light.

BRIEF DESCRIPTION OF DRAWINGS

In the following the safety drive used in a circular-saw is more closelydescribed on the basis of the attached diagrams. The diagrams show:

FIG. 1 a circular-saw bench with safety in a side view;

FIG. 2 a top view of the drive elements of the circular-saw;

FIG. 3 a view of the locking device for the switch lever.

OPTIMUM EMBODIMENT OF THE INVENTION

In the following, only those parts of the shown circular saw will bedescribed which are of importance to the safety drive. Apart from thisthe saw has a known design.

The saw has a frame or a stand 1, only partially shown, which is closedon its upper end by a table plate 2. Beneath the table plate 2 acircular blade 4 is pivoted on a drive shaft 3; FIG. 1 indicates thediameter of rotation only. Positioned behind the blade 4 and beneath thetable plate 2 is a splitting wedge which projects upward and beyond thetable plate 2 and a bonnet fastener 6 for the protecting bonnet 7 of thesaw blade.

A belt pulley 8 is positioned on the opposite end from the circular saw4. Further down in the machine frame there is an electric drive motor 9,whose drive shaft 10 is also equipped with a belt pulley 11. The beltpulleys 8 and 11 are connected and together form a belt drive 13. Thebelt drive 13 has the form of a V-belt drive.

To shut the electromotor 9 on and off there is an encased switch 14,which can be activated with a switch lever 15; the lever 15 projects outof the table stand on the operating side of the circular saw. The switchlever 15 is attached to the control shaft of the switch 14, the shaftsimultaneously forming the pivot 16 for the switch lever 15.

As indicated in FIG. 3, at the point at which it leaves the table standthe switch lever 15 runs through a vertical slot 17, in which it can bemoved back and forth between the upper, engaged position 18 and thelower disengaged position 19. Next to the slot 17 there is a lockingelement 20 which has the form of a two-armed lever 21; one end 22 of thelever 21 crosses the slot 17 like a closing latch and is weighted with aweight 23 at its lower end. The weighted end of the double-armed lever21 is supported by a stop dog 24 which prevents the weighted arm of thelever from leaving its horizontal position and turning in a downwarddirection. From FIG. 3 it can be seen that for the switch lever 15 to beswitched into its disengaged position 19 it must be moved past thelocking element 20, its latch-like end 22 being moved out of line withthe slot 17 through lifting of the weighted end--to then return to itslocked horizontal position due to the effect of the weight 23 after themotion of the switch lever 15 has been completed. Return of the switchlever 15 from its disengaged or closed position 19 is prevented by thestop dog of the locking element 20. If the switch lever 15 is to belifted back up and returned to its engaged position, the locking element20 must be lifted manually at its weighted end until the latch-like end22 opens the slot 17 and makes the movement possible. This arrangementprevents the switch lever 15 from being unintentionally engaged, but atthe same times serves to engage the generator, as will be more closelydescribed below.

The entire drive unit, including the saw blade and protection devices,can be swivelled within the table stand to allow cutting of sectionsthat incline toward the vertical; the unit swivels on horizontal axisbasically running through the saw blade and lying at about the level ofthe table plate.

As can be seen from FIGS. 1 and 2, a generator 25 is attached in thearea of the belt drive 13 by means of a moveable fastener 26; in theembodiment example the fastener 26 consists of a leaf spring. In theexample the generator 25 is a conventional small generator or bicyclegenerator, and for attachment to the leaf spring it is equipped with aclamp, which surrounds the generator's housing. Instead of the drivepulley usually accompanying a small generator, the generator 25 isequipped with a belt pulley. The mobility of the leaf spring fastener 26allows the small generator 25 and its belt pulley 28 to press againstthe drive belt 12, so that the drive belt displaces the armature of thesmall generator during rotation. The small generator 25 is engaged withthe drive belt by means of an activating device, consisting of a switchlever 15 and an activating rod 29, which in the embodiment example alsohas the form of a leaf spring. This leaf spring 29, as an extension ofthe switch lever 15, is attached to the latter on the other side of itspivot point 16, and on its other, angular end it connects with the neckof the small generator. A downward motion of the switch lever 15 intoits off position 19, pushed toward the end of the leaf spring 19attached to the small generator 25, so that it presses the smallgenerator, elastically attached to the leaf spring, and its belt pulleyagainst the upper end of the drive belt 12. The switch lever 15 offposition 19, which represents a locked position, at the same timeassures that the small generator 25 remains equipped with the drive beltuntil the switch lever 15 is deliberately released from locked positionand is returned to on position 18.

When the electromotor is switched off and into locked position 19 bydropping the switch lever 15, the small generator 25 is simultaneouslypositioned against the drive belt 12. The kinetic rotational energystill contained in the saw blade 4 is then extinguished by the smallgenerator 25 acting as generator. The braking effect is adjusted so thatthe regulations are met and the saw blade comes to a stop within 10seconds after the motor is cut off.

The electrical energy produced during the braking action of the smallgenerator 25 can be employed advantageously by operating an electricalwarning device, which releases a signal throughout the saw blade'safterrun. In the embodiment example this warning device has the form ofa warning light 30, located at an easily visible point on the protectingbonnet of the saw blade. The light is fed by a cable line 31. The bonnetholder or splitting wedge can be used as the lead-through area throughthe table plate for at least one of the electrical conductors. A furtherpossibility of mounting a warning light fed by the small generator 25 isshown in FIG. 1, where the grip 32 of the switch lever 15 contains thewarning light 33.

COMMERCIAL USE

The described drive mechanism, employing the most available and cheapeststructural parts and components, assures that the safety requirementscurrently limiting the afterrun time of the saw blade are fulfilled, theenergy produced by the small generator also being used advantageously tooperate a warning device indicating the tool's state of motion duringthe brief afterrun period.

I claim:
 1. A power cutting tool having a rotating blade comprising:anelectric motor with an output shaft; a first belt pulley mounted on saidoutput shaft; a working blade member having an input shaft; a secondbelt pulley mounted on said input shaft; a drive belt secured about saidfirst belt pulley and said second belt pulley and operable to move aboutsaid pulleys; an electric switch having an on position and an offposition and operable to actuate said electric motor in said onposition; a mechanical lever connected to said electric switch andoperable to move said electric switch between said on position and saidoff position; an electric generator having a generator input shaft andoperable to generate an electric current upon rotation of said generatorinput shaft; a third belt pulley mounted on said generator input shaft;an activating rod anchored at one end to said generator and anchored atanother end to said mechanical lever; an electrically actuated warningsignal means electrically connected to said generator and a mechanicallever locking assembly positioned about said mechanical lever permittingsaid mechanical lever to facilely move said electric switch from said onposition to said off position but preventing said mechanical lever fromreadily moving said electric switch from said off position to said onposition, wherein when said electric switch is in said on position saiddrive belt moves about said first and second pulleys and does notcontact said third pulley, but, when said electric switch is shifted tosaid off position said third belt pulley contacts said drive belt andexerts a braking action on said drive belt and a rotational action onsaid generator input shaft producing an electric current from saidgenerator and actuating said warning signal means.
 2. A power cuttingtool as recited in claim 1 wherein:said mechanical switch lever extendsthrough a slotted opening on one side of said tool and wherein saidmechanical lever locking assembly comprises a weighted two-armed leveranchored to said side of said machine, one end of said two-armed leverextending across said slotted opening so that said lever can be brieflydisplaced by said mechanical lever when said mechanical lever is movedfrom a position corresponding to said on position of said electricswitch to a position corresponding to said off position of said electricswitch and another end of said two-armed lever serving as a stop meanspreventing said mechanical lever from being readily shifted in theopposite direction.